They are mostly life long dims who are indoctrinated into socialism and communism and all of the evil that comes with them.
“What’s in it for them? Is it just a giant MSM circle jerk or what is it?”
The “progressives” have been actively taking over the media since at least the 1950’s if not before. They select “progressives” in their hiring and those who do not have the correct politics never get promoted, get fired, and/or are ostracized.
They view government power as the supreme good and are perfectly willing to give up profits “for the cause”.
What is in it for them is the belief in an agenda greater than themselves. They see government power as god on earth.
One of my friends got a journalism degree about 1974, then he worked his way up into a major market. Ten years later, he finally quit in disgust. He said: “If I do a story on Republican corruption, it is front page news. If I do one on Democrat corruption, it gets spiked.”
Occasionally, a rare “progressive” will be converted to conservatism, or at least a critique of “progressivism” after attaining enough prominence that they cannot easily be fired. They are then villified and attacked. That is what has happened with John Stossel, Mike Royko, and David Horowitz.